1. Field of the Invention
This invention relates to a gas concentration-detecting device for detecting the concentration of gas, such as air or the like, in oil by detecting a capacitance (electrostatic capacitance) between a plurality of electrodes arranged in the oil.
2. Description of the Prior Art
Generally, in a lubricating system of an industrial machine, when the concentration of gas, such as air or the like, in oil increases, the lubricating performance of the oil decreases to cause metal contact or the like, and in the worst case, seizure of sliding portions and rotating portions is brought about. Further, in an actuating system of the industrial machine as well, if the concentration of gas in oil increases, there is a fear that hydraulic pressure decreases to make an actuating member of the system inoperative or cause a delay in response or an erroneous operation of the member. To prevent such inconveniences, it is desirable to periodically detect and check the concentration of gas in oil. Conventionally, as gas concentration-detecting devices for detecting the concentration of gas in oil, there have been proposed (1) one for calculating the concentration of gas by sampling a predetermined amount of oil from a lubricating system or an actuating system of a machine, and determining a volume ratio of gas contained in the oil, (2) one for providing a bypass passageway bypassing an oil path and having a volumeter or a weightometer arranged therein, and calculating back the concentration of gas based on a density per volume of the oil determined by using the volumeter or the weightometer, and (3) one for detecting the concentration of gas by arranging a capacitance sensor comprised of two electrodes in oil and detecting a capacitance between the two electrodes.
The above conventional gas concentration-detecting device (1) is constructed such that sampling of the oil is necessary as described above. However, it is impossible to carry out sampling operations in a manner coping with changes in the gas concentration, so that transient changes in the gas concentration cannot be detected in real time. Therefore, when the gas concentration-detecting device is applied to a lubricating system or an actuating system, if the gas concentration sharply increases in a short time period, the device cannot detect the change, which sometimes causes the above-mentioned inconveniences. Further, the amount of oil is changed by sampling thereof, causing a change in the state of a mixture of gas and oil, which can increase an error in the detection of a gas concentration. Further, in the gas concentration-detecting device (2), oil is caused to flow through the bypass passageway. The concentration of gas in oil present within the passageway is sometimes different from an actual concentration of gas in oil present in the oil path. In such a case as well, an error in the detection of a gas concentration is increased. Furthermore, the provision of the bypass passageway and the volumeter or weightometor results in an increased size of the system and increased manufacturing costs of the industrial machine.
On the other hand, the gas concentration-detecting device (3) is generally constructed such that a relatively small-sized capacitance sensor is directly inserted into or mounted in an oil path. Further, the output from the capacitance sensor is highly responsive to changes in the gas concentration, so that it is possible to detect transient changes in the gas concentration in real time. Therefore, the gas concentration-detecting device (3) is free from the inconveniences with the above gas concentration-detecting devices (1) and (2). In the device (3), however, the output from the capacitance sensor is susceptible to a change in oil temperature and varies with this change. Further in a region of the oil temperature beyond a predetermined value, the higher the oil temperature becomes, the more significant drift of the sensor output is caused (in this specification, the term xe2x80x9cdriftxe2x80x9d of an output from a capacitance sensor is used to mean deviation of an actual value of the output from the capacitance sensor obtained when the capacitance sensor is placed e.g. in 100% oil having no gas mixed therein, from an assumed value of the output from the capacitance sensor which the capacitance sensor is to output assuming that the output from the sensor placed in the 100% oil varies with a predetermined difference from an output from the sensor placed in 100% gas). Therefore, when the output from the capacitance sensor suffers from a significant drift as described above, gas concentration detection accuracy of the device is lowered. Further, when the oil is degraded as well, the output from the capacitance sensor suffers from a drift, resulting in a lowered gas concentration detection accuracy of the device.
It is an object of the invention to provide a gas concentration-detecting device which is capable of detecting the concentration of gas in oil with accuracy, irrespective of the temperature of oil, and even when the oil is degraded.
To attain the above object, the present invention provides a gas concentration-detecting device for detecting a concentration of gas in oil, comprising:
a plurality of electrodes for being arranged in the oil such that the electrodes are opposed to each other and spaced from each other by a predetermined distance; and
gas concentration detection means for outputting a signal indicative of the concentration of the gas in the oil, the signal being dependent on a capacitance between the electrodes,
wherein the predetermined distance between the electrodes is set to a value equal to or larger than a predetermined value at which an amount of drift of the signal output from the gas concentration detection means dependent on variation in a temperature of the oil starts to gradually decrease as a distance between the electrodes is increased.
According to this gas concentration-detecting device for detecting a concentration of gas in oil, the gas concentration detection means outputs a signal indicative of a detected concentration of gas in oil and dependent on a capacitance between the plurality of electrodes. Hence, it is possible to detect the concentration of the gas in real time, while coping with the change in the concentration of the gas. Generally, a signal indicative of a capacitance between two electrodes in oil varies with the temperature of the oil, and an amount of drift occurring at the time is reduced as the distance between the two electrodes is increased. More specifically, as the distance between the two electrodes is decreased, the amount of drift has a characteristic that it sharply decreases before the distance becomes equal to a predetermined value, and gradually decreases as the distance is increased beyond the predetermined value. In the present invention, attention is paid to the above characteristic of the amount of drift, and the predetermined distance between the two electrodes is set to a value equal to or larger than the predetermined value at which the amount of drift of the signal output from the gas concentration detection means dependent on variation in the temperature of the oil starts to gradually decrease as the distance between the electrodes is increased. This makes it possible to effectively minimize the amount of drift of the signal, and thereby detect the concentration of gas in oil with accuracy irrespective of the temperature of oil.
Preferably, the gas concentration-detecting device further includes oil temperature detection means for detecting the temperature of the oil, and correction means for correcting the signal output from the gas concentration detection means, according to the temperature of the oil detected by the oil temperature detection means.
According to this preferred embodiment, by using the correction means, the signal output from the gas concentration detection means containing the above amount of drift can be corrected, according to the temperature of the oil detected by the oil temperature detection means. This makes it possible to compensate for the influence of a change in the temperature of the oil on the signal, thereby enhancing the accuracy of detection of the gas concentration.
More preferably, the predetermined distance between the electrodes is set to a value within a range between the predetermined value and a second predetermined value which is larger than the predetermined value, the range being determined based on a characteristic of the amount of drift of the signal output in relation to degradation of the oil.
Generally, the signal indicative of a capacitance between two electrodes in oil has a characteristic that it is drifted by degradation of oil, and the amount of the drift simply increases as the distance between the two electrodes is increased, so long as the distance electrodes is equal to or larger than a certain value. In this preferred embodiment, attention is paid to the above characteristic of the amount of drift, and the predetermined distance between the electrodes is set to a value within a range between the predetermined value and a second predetermined value which is larger than the predetermined value, the range being determined based on the characteristic of the amount of drift of the signal in relation to degradation of the oil. Therefore, if the second predetermined value is set to a value which can sufficiently minimize the amount of drift caused by degradation of oil, it is possible to suppress not only a drift of the signal output from the gas concentration detection means due to a change in oil temperature, but also a drift of the same due to degradation of oil. This makes it possible to detect the concentration of gas in the oil with higher accuracy.
Preferably, the gas concentration detection means comprises a capacitance amplifier which applies a predetermined voltage to the electrodes and detects a voltage generated in response to the application of the predetermined voltage, dependent on the capacitance between the electrodes to output the voltage as the signal.
More preferably, the correction means includes a lookup table formed by correlating actual measurement values of two kinds of the voltage output from the capacitance amplifier in respective cases of application of the predetermined voltage carried out when the concentration of the gas in the oil is 100% and when the concentration of the gas in the oil is 0% to corresponding values of the temperature of the oil assumed when the application of the predetermined voltage is carried out, and interpolation calculation means for carrying out interpolation calculation by using two values of the actual measurement values of the respective two kinds read from the lookup table according to the temperature of the oil detected by the oil temperature detection means, and an actual value of the voltage output from the capacitance amplifier.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.